1. Field of the Invention
The present invention relates to particulate-filled polymer matrix composite materials for circuit boards and methods of manufacturing the same. More particularly, this invention relates to filled fluoropolymer matrix composite films for circuit board substrates suitable for casting as thick films without cracking.
2. Brief Description of the Related Art
Electrical circuit boards generally comprise a conductive layer adhered to a dielectric substrate layer. The conductive layer is generally a metal such as copper, and the substrate layer is a dielectric polymer or polymer composite. Depending on the application for the circuit board, many different polymers, including phenolics, polyesters, epoxies, polyimides, and fluoropolymers are used. For high frequency electronic applications, such as radar, satellite communications, and cellular telephone systems, fluoropolymers such as poly(tetrafluoroethylene) (PTFE) are often the preferred matrix polymer due to their unique combination of excellent electrical properties and chemical resistance, as well as high temperature resistance. However, due to their high coefficient of thermal expansion relative to that of the conductive cladding, pure fluoropolymers are rarely used as a circuit substrate without combination with woven or random glass reinforcement, or particulate filler.
Woven glass-reinforced fluoropolymer matrix composites are generally made by a dip coating process, that is, by dip coating the woven glass material in an aqueous fluoropolymer dispersion. One drawback of the standard dip coating process is that multiple passes are required to achieve fluoropolymer contents of greater than about 25 weight percent in the final composite. Attempts to make higher fluoropolymer content composites in a single pass on a dip coating line result in severe cracking of the fluoropolymer coating upon drying.
Random glass-reinforced or particulate-filled fluoropolymer matrix composite films suitable for use as circuit board substrates are generally made by known paper-making, skiving, casting, melt extrusion, or "paste extrusion and calendering" processes. Fluoropolymer films produced by paper-making processes require random fiber reinforcement and are limited to thicknesses greater than about 2 mil.
Melt extrusion would be a preferred method for making particulate filled composites, but PTFE can not be melt extruded due to its extraordinarily high melt viscosity. The high melt viscosity of other neat fluoropolymers also complicates the production of fluoropolymer films by melt extrusion. Certain fluorocopolymers are known which provide lower melting temperature and lower melt viscosity at extrusion temperatures, for example, copolymers of tetrafluoroethylene with hexafluoropropylene (FEP) or with ethylene. The introduction of fillers further complicates the melt extrusion of fluoropolymers. In the presence of certain fillers, especially at high filler loading level, the melt processability of the melt extrudable fluoropolymers is rapidly degraded due to the increase in melt viscosity associated with the presence of the filler, or due to filler-catalyzed thermal degradation of the polymer matrix.
Accordingly, paste extrusion and calendering is a preferred method of making particulate-filled fluoropolymer films for circuit board substrates. A method of making highly filled PTFE composite materials which exhibit excellent physical and electrical properties by paste extrusion and calendering is set forth in commonly assigned U.S. Pat. No. 4,849,284 to Arthur et al., the disclosure of which is incorporated herein by reference in its entirety. Another preferred method of making a filled fluoropolymer circuit substrate is by casting from aqueous dispersion. This method is particularly convenient and cost-effective for the manufacture of thin films of fluoropolymer is further described in commonly-assigned U.S. Pat. No. 5,312,576 to Swei et al., also incorporated herein by reference in its entirety.
The casting method of Swei et al. comprises adding particulate filler to aqueous PTFE dispersion, adding a surfactant to facilitate wetting of the filler particles, and a viscosifying agent to retard setting of the mixture, and then casting the mixture onto a carrier film. After casting, the films are baked to remove volatile materials, sintered, and removed from the casting substrate. The cast films have a variable thickness in the range from about 0.5 to 2.5 mil, and are intended for use as substrates for thin digital circuits. Thicker sheets (up to about 5 mils) may be cast by this method by the incorporation of a 40 foot, low temperature platen drier between the casting step and the baking and sintering oven.
Thus, while Swei et al. describes casting methods suitable for films having thicknesses in the range of about 0.5 to 5 mils, there remains a need for methods of forming thicker particulate-filled substrates in the range from about 10 to about 100 mils, particularly for use with microwave circuit boards. Manufacture of thick substrates currently requires lamination of a stack of multiple sheets, each sheet having a thickness of less than 6 mils, and each made by a single pass through the manufacturing line, which adds considerably to the expense of manufacture.
An additional problem associated with casting is that the carrier films onto which the dispersion is cast have a finite lifetime, degrading upon repeated exposure to high temperatures. A cast sheet achieving just twice the thickness of prior art sheets during a single pass would effectively halve the amortized cost of the carrier on a square foot per mil basis. Finally, attempts to cast fluoropolymer composite films having a lower filler content (less than about 40 volume percent) by the method of Swei et al. results in cracking of the film during drying under conventional conditions. Similar attempts to cast films having a thickness of at least about 6 mils of higher particulate content composites by the method of Swei et al. can result in cracking of the film during drying under conventional conditions. The resulting cracking is an impediment to the processing and use of thicker fluoropolymer composite sheets.
Accordingly, there remains a need in the art for a method and formula for forming fluoropolymer composite sheets having a thickness of at least about 10 mils, and which can be manufactured without cracking when the composite sheets are dried at an economical rate, i.e., at greater than 4 feet per minute (fpm). There is also a need in the art for fluoropolymer mixtures which will minimize the number of passes required to form thick layers on woven glass.